1. Field of the Invention
The invention relates generally to the field of marine seismic survey systems. More particularly, the invention relates to paravanes used to exert controllable lateral force on a seismic streamer array.
2. Background Art
Marine seismic survey systems are used to acquire seismic data from Earth formations below the bottom of a body of water, such as a lake or the ocean. Marine seismic survey systems known in the art typically include a seismic vessel having onboard navigation, seismic energy source control, and data recording equipment. The seismic vessel is typically configured to tow one or more seismic sensor “streamers” (long cables with sensors at spaced apart locations along the cable) through the water. At selected times, the seismic energy source control equipment causes one or more seismic energy sources, which may be towed in the water by the seismic vessel or by another vessel, to actuate. Signals produced by the sensors in the one or more streamers ultimately in response to energy from the seismic source are conducted by electrical, radio and/or optical telemetry to the recording equipment, where a record indexed with respect to source actuation time is made of the signals produced by each sensor (or groups of such sensors). The recorded signals are later interpreted to infer the structure and/or composition of the Earth formations below the bottom of the body of water.
As explained above, the one or more streamers are in the most general sense long cables that have seismic sensors disposed at spaced apart positions along the length of the cable. A typical streamer can extend behind the seismic vessel for several kilometers. More recently, marine seismic acquisition systems have been designed that include a number of such streamers towed by the seismic vessel parallel to each other. A typical multiple streamer system includes a plurality of “lead-in cables” each coupled to a forward end of one of the streamers. The lead-in cables are used to deploy the streamers from the seismic vessel and to maintain the streamers at a selected distance behind the vessel.
The streamers are fixed near their forward ends to a spreader cable or “super wide” cable. The spreader cable extends in the water transversely to the direction of motion of the vessel, and when maintained in the correct tension, substantially fixes the relative lateral positions of the forward ends of the streamers. The spreader cable is maintained in tension by a device coupled to each end of the spreader cable called a paravane.
The paravanes include diverters or similar suitably-shaped deflecting plates that redirect the motion of the water past the paravane laterally to produce some amount of lateral force ultimately applied to the spreader cable. The lateral force exerted by the paravanes is related to the shape and orientation of the deflecting plates and the speed of motion of the paravanes through the water. One issue of concern to seismic survey system operators using paravanes known in the art is that the lateral force exerted by the outermost paravane when the vessel turns can be excessive at ordinary towing speeds because the outermost paravane will be moving considerably faster than the vessel, depending on the lateral distance between such paravane and the centerline of the vessel. Thus, using paravanes known in the art, it is frequently necessary to limit the vessel speed during turns, thus reducing the efficiency with which a survey can be performed. Where there are water currents that move in a direction along the direction of motion of the seismic vessel, the amount of lateral force exerted by paravanes known in the art will increase with current flow opposite the vessel direction because of the increased water velocity past the paravane, or will decrease with water current flow in the same direction as the seismic vessel motion because of decreased water velocity past the paravane. Where there are water currents moving transversely to the direction of motion of the seismic vessel, the seismic streamer array may be moved laterally in a manner that is difficult for the system operator to compensate or control.
It is also known in the art that the most suitable configuration for the array of cables that ultimately couples the paravanes to the spreader cable (called a “bridle”) and/or tow rope may vary depending on the particular paravane used, and on actual vessel motion conditions. In the event the bridle actually used in any survey is not optimal for the existing equipment configuration and survey conditions, it is frequently necessary to retrieve the paravane and reconfigure the bridle. Such retrieval and reconfiguration operations can be costly and time consuming.
There continues to be a need for improved structures for paravanes and bridles to increase seismic survey efficiency.